A major problem encountered in the treatment of various hydrocarbon charge stocks is the phenomenon recognized as and descriptively called fouling. This phenomenon is manifested in the form of deposits which frequently form on the metal surfaces of the processing equipment and tend to decrease the efficiency of the intermediate processing operations. The results of fouling appear in the form of heat transfer loss, increased pressure drop and a loss in throughput rate. Fouling also increases the safety risks associated with operating a chemical process. It is therefore a beneficial practice to inhibit the build-up of deposits in processing equipment that would otherwise reduce capacity and overall plant efficiency.
Ethylene dichloride (EDC) is produced by two methods; direct chlorination and oxychlorination. Both chlorination reactions are combined in most production plants to produce EDC by what is known as a balanced process. Hydrochloric acid produced in the direct chlorination process is used as a feedstock in the oxychlorination process.
Oxychlorination is a catalytic process by which ethylene, oxygen and hydrogen chloride are combined to produce ethylene dichloride and water. Any time ethylene, oxygen and a catalyst are combined, the possibility of oxidation exists. Acetaldehyde is an oxidation product of ethylene, and it is a minor by-product of the oxychlorination process.
From the reactor the effluent (crude EDC) is washed with water and caustic to remove as much hydrogen chloride as possible from the stream. It is during the caustic wash step that acetaldehyde undergoes a base-catalyzed polymerization.
Generally, the basic washing entails contacting the ethylene dichloride with an aqueous basic solution in a wash tower to remove hydrogen chloride therefrom. The conditions in the wash tower are conducive for condensation reactions of many enolizable aldehydes (such as acetaldehyde) contained therein.
The term "crude ethylene dichloride" refers to unpurified EDC which leaves the chlorination of oxychlorination units. Crude EDC also refers to the feed stream for the EDC Tar Still. These units may be considered distillation separation units which separate the crude EDC stream into an overhead stream of purified EDC and a bottoms stream of EDC, 1,1,2-trichloroethane, hexachloroethane, hexachlorobenzene, with fouling amounts of chlorinated or oxychlorinated polymeric material. The fouling in the bottoms is severe at EDC levels of below about 30 wt %, and particularly severe at EDC levels of below about 30 wt %, and particularly severe at EDC levels of 20 wt % or below. At high EDC levels in the bottoms, EDC acts as a solvent for the fouling materials.
Materials from the direct chlorination unit often contain not only chlorinated products, but also iron complexes; the iron typically comes from the catalyst (e.g. ferric chloride or tetrachloroferrate salts) used in the reaction or from corrosion of the equipment used in the process. These unwanted species, i.e. extraneous organic chloride/oxygenates and iron complexes, are typically removed through a series of aqueous washings and distillation columns. Columns are used to concentrate and remove the tars (i.e. heavies) formed either during the chlorination step or downstream of the reactors. Tars are generally high boiling polychlorinated by-products with poorly defined compositions. Material lighter than ethylene dichloride (b.p. 83.degree. C.) is also removed through fractionation. After prolonged usage, both types of columns eventually foul due to the accumulation of non-volatile by-products.
Some of the by-products are likely due to unwanted oxidation (e.g. chloral and other oxygenated by-products are known to form). Oxygen is frequently an impurity in chlorine, and oxygen is also used in the oxychlorination process. Infrared analysis of non-volatile components from a heavy ends removal column has revealed that carbonyl moieties do indeed exist.
Serious fouling occurs in the various units handling the liquid EDC. For example, in the primary EDC Recovery Unit, fouling occurs in the distillation trays and the transfer facilities, particularly the retort furnace. EDC fouling is particularly serious in the liquid phase of EDC in the primary EDC Recovery Unit, the EDC Recovery Tar Still, and the EDC Recycle Tar Still.
It is not uncommon for these units to foul within periods of time as short as a few days. If it were possible, by the use of additives, to increase the time before fouling caused shut downs for cleaning a valuable contribution to the art would be made. Not only would run-length of the unit be increased, but there would be a reduction in waste and lower potential exposure to those who clean the unit.
Among the methods for inhibiting carbonyl fouling in caustic scrubbers are U.S. Pat. No. 4,673,489 wherein hydroxylamine and its hydrochloride and hydrogen sulfate salts have been used to inhibit polymer formation caused by condensation reactions of aldehydes contained in caustic scrubber units; U.S. Pat. No. 4,952,301 wherein ethylenediamines and water soluble salt forms thereof have been used to inhibit carbonyl based fouling, particularly aldehyde fouling, that often occurs during caustic scrubbing of liquid or gas phase hydrocarbon streams in the base wash unit; U.S. Pat. No. 5,264,114 wherein the use of amine compounds to inhibit the deposition of foulants during caustic washing is disclosed; U.S. Pat. No. 5,160,425 wherein carbohydrazide has been disclosed as useful for inhibiting polymeric fouling deposits during the caustic scrubbing of pyrolytically-produced hydrocarbons contaminated with oxygen-containing compounds in, hydrazides for the same purpose have been disclosed in U.S. Pat. No. 5,288,394; and U.S. Pat. No. 5,194,143 wherein an acetoacetate ester is used in a method for inhibiting fouling during caustic washing of hydrocarbons. Amide condensation products of monocarboxylic acids and aliphatic polyamines for the same purpose were disclosed in U.S. Pat. No. 3,364,130. Additionally, U.S. Pat. No. 5,220,104 discloses the use of percarbonate salts for the same purpose. Yet none of these references disclose the novel treatment disclosed herein, and the majority would be unsuitable for the ethylene dichloride caustic tower environment.
Antifoulant dispersants have been described for the prevention of fouling in ethylene dichloride streams within the distillation units of the ethylene dichloride manufacturing process. For example, U.S. Pat. Nos. 5,240,469 and 5,324,393 disclose the use of a composition comprising an acrylate ester containing C.sub.4 -C.sub.22 alcohol esters and amino alcohol esters, phenylene diamine and a heavy aromatic solvent; and U.S. Pat. No. 5,110,997 discloses use of an acylated amine, a magnesium sulfonate or a blend of both.
However, dispersants such as those described above as useful for distillation tower treatment would not also serve as antifoulants for caustic tower treatment. This is so because the caustic tower, treated by the antifoulants disclosed herein, is located in front of the distillation units in an ethylene dichloride manufacturing facility. The bottom of the caustic tower contains aqueous sodium hydroxide, and the overhead steam from the caustic tower contains water and crude ethylene dichloride. The caustic tower (also referred to as a spent caustic wash system) is an aqueous environment. This stream goes next to the distillation section, wherein water and light hydrocarbons are removed in the dehydration tower. The bottom of the distillation unit does not contain any water, but rather is organic in nature. Therefore the environment in the caustic tower is different than the environment within the distillation unit, and the ensuing problems and their solutions generated within the distillation tower will be different from those generated within the caustic tower.
Moreover, inhibition of corrosion in halocarbon systems with succinic acid derivatives is disclosed in U.S. Pat. No. 4,422,953. Since corrosion does not occur in caustic towers under normal operating conditions, corrosion inhibitors would not be used in the caustic towers of either ethylene plants or ethylene dichloride plants.
A method for inhibiting ethylene dichloride process stream fouling by preventing polymerization due to aldol condensation of aldehydes such as acetaldehyde that does not interfere with overall plant operations or in the operation of individual process units would therefore be a highly desirable advancement in the art of ethylene dichloride manufacture.